Our ESR study of dimerization of metal porphyrins, which covered so far, a number of Cu(II) and Fe(III) complexes, is now extended to a Fe(II) complex. The result is especially significant in view of the role played by binuclear complexes in dioxygen activation in biological systems. We have started application of spin labeling ESR method to the study of red blood cell deformability in an attempt to obtain rheological information of the cell. Preliminary results show some discrepancy from the behavior projected from simple fluid mechanical arguments, and call for further investigation.